Mitochondria are essential organelles in nearly all eukaryotic cells. These membrane bound structures contain their own DNA, and maintenance of this DNA is vital for normal cellular function. Mutations in the human mitochondrial genome have been implicated in many diseases, including neuromuscular disorders, heart disease, and diabetes. Moreover, the accumulation of mutations in the mitochondrial genome has been shown to contribute to aging and aging-related disorders. Mutations in DNA occur as a result of errors in DNA synthesis and repair processes acting on damaged DNA. Ultraviolet (UV) light is a DNA damaging agent which efficiently induces mutations in mitochondrial DNA of both yeast and humans. The mechanisms involved in the repair and maintenance of mitochondrial DNA in response to UV exposure are poorly understood. The studies proposed here address fundamental processes involved in mitochondrial genome maintenance after exposure to UV light. The specific aims of this proposal are I. to characterize the REVersionless mutants in UV-induced mutagenesis of mitochondrial DNA using genetic reporters allowing for the determination of the frequency of spontaneous and induced mutations of different types, and II. to determine the role of the heterotrimeric Rad17p/Mec3p/Ddc1p checkpoint clamp of Saccharomyces cerevisiae. Both molecular and genetic approaches will be used to determine the function of these proteins in the prevention of mitochondrial dysfunction. Relevance: Mitochondria contain their own DNA. This mtDNA is prone to DNA damage by chemical as well as enviornmental agents. One such agent is ultraviolet light, which causes lesions in the DNA. As the cell attempts to repair or bypass these lesions, mutations are produced. It has been shown that accumulation of mutations in mtDNA contributes to many diseases and disorders. Studying recovery of mtDNA after UV exposure in the budding yeast may shed light on how these processes occur in human mtDNA.